Numerous processes for the production of isocyanurate polymerization products are known. These are predominantly the polymerization products of polyisocyanates in basic catalysts. Insoluble end products with a plastic-like character are obtained in these processes when polymerization is carried through to completion, and are accompanied by the formation of a plurality of isocyanurate rings. Alternatively, soluble NCO-group-containing polymerization products of isocyanurate structure (which are of higher functionality by comparison with the starting isocyanates) are obtained when polymerization is prematurely terminated, for example, by neutralizing the catalysts. These relatively high molecular weight, soluble polyisocyanates of isocyanurate structure are widely used in polyurethane chemistry, for example, as crosslinkers in the production of lacquers, or in the production of foams as recommended in German Patent Nos. 1,022,789 and 1,027,394. Over recent years, foam applications have acquired considerable significance because it is now known that highly elastic, flameproof polyether foams may be obtained with such polyisocyanates of isocyanurate structure and relatively high functionality, provided that production of the foams is carried out in the absence of conventional foam stabilizers of the organosiloxane-hydroxy alkylene block copolymer type (German Auslegeschrift No. 1,929,034). In view of the considerable significance of this discovery, it may readily be appreciated why increased interest is also being shown in processes for the production of polyisocyanates of isocyanurate structure. Safe control of the exothermic polymerization reaction on an industrial scale is a particularly important factor.
One feature of conventional processes for the production of polyisocyanates of isocyanurate structure is the need for the premature termination of the polymerization of the starting polyisocyanates, catalyzed by basic catalysts. Termination is generally obtained by inactivating the catalysts by the addition of acid-reacting substances or alkylating agents. Unless polymerization is terminated, the polymerization reaction ultimately results in the formation of insoluble and, hence, unusable end products. In many cases, the uncontrolled, exothermic polymerization reactions also result in dangerous, uncontrollable increases in temperature which may ultimately initiate secondary reactions, such as carbodiimide formation followed by urethane imine formation, so that totally unusable end products are obtained.
Another disadvantage of the production of polymerization products by trimerizing isocyanates are the incubation times occasionally observed before the onset of the trimerization reactions. This is the case, for example, with the process described in German Patent No. 1,106,767. If, in the process described therein, 0.66 part of triethylene diamine and 1.34 parts of propylene oxide as co-catalyst are added to a solution of 100 parts of phenyl isocyanate in 100 parts of acetone, an exothermic trimerization reaction is only observed after a prolonged incubation time of approximately 70 minutes at 30.degree. C. In practice, any polymerization reaction which begins immediately after the addition of catalyst and which progresses steadily is preferred to a polymerization reaction involving an incubation period, followed by a highly exothermic reaction, because of the greater operational reliability.
One very versatile process for the production of polyisocyanates of isocyanurate structure is described in German Patent No. 1,013,869. In this process, monomeric organic monoisocyanates and/or polyisocyanates are heated in the presence of small quantities of tertiary amines and carbamic acid esters mono-substituted on the nitrogen atom. However, the need for polymerization to be prematurely terminated by the addition of acid-reacting substances is still essential in this process, since tertiary amines such as hexahydrodimethyl aniline or per-methylated diethylene triamine are used.